Aboard a No. 6 local train in Manhattan, Weill Cornell researcher Christopher Mason patiently rubbed a nylon swab back and forth along a metal handrail, collecting DNA in an effort to identify the bacteria in the New York City subway.

In 18 months of scouring the entire system, he has found germs that can cause bubonic plague uptown, meningitis in midtown, stomach trouble in the financial district and antibiotic-resistant infections throughout the boroughs.

Frequently, he and his team also found bacteria that keep the city livable, by sopping up hazardous chemicals or digesting toxic waste. They could even track the trail of bacteria created by the city’s taste for pizza—identifying microbes associated with cheese and sausage at scores of subway stops.